1. Field of the Invention
This invention relates to the field of power sources, and particularly to power sources which convert motion into energy.
2. Description of the Related Art
Recently passed government legislation mandates that certain types of vehicles provide on-board, real time tire pressure monitoring—i.e., a system which monitors the air pressure in all vehicle tires. This requires an electronics package to be mounted on or inside the tire or wheel, which is small enough so as to not cause any major weight imbalance. The package would include an air pressure sensor and a means of wirelessly transmitting sensor output data to the vehicle driver, or of storing the data for later retrieval.
The electronics package must be powered, typically by a battery. Several approaches have been considered for the power source. In one approach, a conventional battery is mounted inside the tire, secured to the wheel with a metal band. The system operates until the battery reaches the end of its useful life, which is likely to happen before the tire needs replacing. As such, the battery must be replaced, which requires that the tire be removed from the wheel. This periodic battery replacement is costly.
A second approach employs a rechargeable battery placed within the tire, which is charged via inductive coupling with a power source outside of the tire. For the coupling to be effective, the inductive power source must be mounted very close to the tire. This system also has several drawbacks. The close distance required between tire and charger may pose a safety concern. In addition, substances such as mud or snow can accumulate on the tire or inductive power source and impede the performance of the charging system.
Another approach has the electronics package wired to the tire valve, with the battery and/or electronics mounted on the tire's valve stem and monitoring the tire pressure via the valve. Unfortunately, this approach makes the external unit an easy target for theft.